Conventionally, a pressure sensor using a crystal resonator is known as a detection element used in gauges such as a water pressure gauge, an air gauge, and a differential pressure gauge. The crystal resonator includes an electrode pattern formed on a plate-like quartz substrate. By taking advantage of the characteristics of the crystal resonator that its resonant frequency varies when pressure is applied in the axial direction, the pressure sensor is made to detect pressure changes.
With the pressure sensor using the crystal resonator, the resonant frequency varies in approximate proportion to an applied pressure (showing a quadratic curve). Thus, by compensating the relation between the frequency variation and the applied pressure by using a quadratic equation, high-accuracy pressure measurement becomes possible.
However, in an attempt to realize the high-accuracy pressure measurement, problems occur that the structure becomes complicated and the manufacturing cost becomes higher. That is, a conventional pressure sensor disclosed in Patent Document 1 includes: a case provided with a first and a second pressure input orifices respectively disposed on two wall surfaces facing each other and maintained in a vacuum or inert atmosphere on the inside, a first electro-deposited bellow whose one end opening side is fixed on a first wall surface and a second electro-deposited bellow whose one end opening side is fixed on a second wall surface, a force transmitting member interposed between the other ends of both bellows, a resonator supporting member joined to the force transmitting member via a flexure hinge, and a plate-like crystal resonator whose both ends are each supported by the force transmitting member and the resonator supporting member. The resonator supporting member is fixed, at its base, on the inner wall of the case and is equipped with the flexure hinge (a pivot) at the part joined to the force transmitting member.
With this conventional example, it is necessary to use the electro-deposited bellows having a very small spring constant to convert gas and liquid pressure into a mechanical force and the flexure hinge with slim constricted parts. Because of high cost of these parts, the cost of the whole product becomes inevitably high.
Further, for price reduction, a system using inexpensive molded bellows and the force transmitting member having no flexure hinge is proposed. However, the pressure sensor of this type has a disadvantage in that, as the applied pressure increases, a bending stress component is applied to the crystal resonator in addition to the axial force. Thus, a linear frequency variation (the quadratic equation) cannot be obtained, and a relation of a cubic curve having a third-order coefficient is produced. Thus, the method of compensating the relationship between the frequency variation and the applied pressure by the quadratic equation suffers a disadvantage that it decreases the accuracy.
In order to solve the problems, the applicant proposed a pressure sensor in Patent Document 2. The sensor includes an air-tight case in which two inexpensive molded bellows having a cylindrical shape are disposed in series or coaxially and a crystal resonator is supported by a pedestal disposed between the ends of both bellows.
However, it has been known that the pressure sensor of this type has a disadvantage in that it has a weak strength against an impact from a direction (an X-axis direction) perpendicular to the axial direction of the cylindrical bellows.
Then, the applicant proposed a double-ended tuning fork type piezoelectric resonator having a thin plate shape in Patent Document 3. In the resonator, two vibration portions having a narrow width strip shape are disposed in parallel with a slit interposed there between, and each of the both ends of the vibration portions is supported by a supporting member having a large area with a connecting member having a small area that is interposed therebetween. The double-ended tuning fork type piezoelectric resonator used in a pressure sensor as a pressure sensitive element allow sensors to be downsized, simplified, achieve low costs, and have high sensitivity since it does not need complicated mechanical elements such as bellows and oscillating arms.
The double-ended tuning fork piezoelectric resonator has a structure in which the connecting member and the supporting member are respectively provided in series at the both ends of two vibration portions. Thus, it is necessary to hold the double-ended tuning fork piezoelectric resonator in a stretched condition in an air-tight space formed in two quartz diaphragms. This may complicate a supporting structure and increase assembly man-hours.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 56-119519.    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2005-121628.    [Patent Document 3] Japanese Examined Patent Application Publication No. 7-109970.